Quadrivalent formation in a tetraploid chicken oocyte.

Synaptonemal complex analysis of an exceptional tetraploid oocyte from a diploid chicken heterozygous for the MN t (Z;1) rearrangement was performed by electron microscopy of a spread preparation. Ten separate quadrivalents (26% of the chromosomal axes) were analyzed, as well as 50 autosomal bivalents. All the axes less than 2.5 microns in length formed bivalents (38) only, while axes in the 2.5-4.2 micron range formed 5 quadrivalents and 12 bivalents. The longer, separate axes formed quadrivalents only. Partner switches in excess of one were documented. The two identical W chromosomes paired only at the ends of their short arms. Quadrivalent formation may require a threshold length (2.5 microns), at least in this species. The tip of the short arm of the W chromosome may be a pairing initiation point, and it corresponds to the region associated with a localized recombination nodule previously described in diploid oocytes.